bug#20272: Support reproducible builds

Jan Nieuwenhuizen writes: I have been building quite some versions of Guile the past week. There re a couple of .go files that keep giving me problems wrt nondeterminism: web/server/http.go (define *timer* (gettimeofday)) seemed suspect, but seems harmless. rnrs/arithmetic/fixnums.go

bug#24909: Recent async changes in master cause segfault

Looks like 3f23688 fixes it. -Dale